Katana ZERO: Unlock the Secret Boss Fight

AntMag.NET

Katana ZERO has a secret boss fight that you can find if you perform several steps. Many of these steps change conversation options and can prevent options that give you achievements. For this reason, I suggest playing through the game once using the main guide, making choices as you like (with only a few exceptions) before returning for the secret boss. It only takes about 10-15 minutes to set up once you're familiar with the game.

Finding the Secret Boss

There are several steps that are required to unlock the secret boss. It's recommended that you perform these steps in order because some of them require you to have performed a previous step for the correct dialog options to occur. However, if you appear to have completed all of the steps but the boss will not appear, it's ok to just try each step individually.

NOTE: There appears to be a minor glitch (or maybe it's just a version difference) on Nintendo Switch with step one. If you simply hang up the phone several times, go to the next screen (which theoretically saves that progress), and then quit out, it seems like the progress is NOT saved despite this working for other steps and apparently working on PC for this particular step.
For that reason, on step one, I recommend Switch players complete the entire level after the phone call. As far as I've seen, PC players should be able to quit out as soon as the game saves, which usually happens when you go to the next screen. It's a short level, however, and it might be worth completing it as a precaution.

On the factory level, proceed until you receive a phone call. Keep hanging up the phone until your quick response time gets very short.
During the Psychiatrist visit before the Hotel mission, berate the psychiatrist and demand your medication.
At the hotel, irritate the receptionist until she yells at you. Complete the entire mission and kill the two cops at the end.
Like with #2, during the Psychiatrist visit before the Club mission, berate the psychiatrist and demand your medication.
Toward the end of the Club mission, you'll get a phone call. Hang up on them. Go into the next room and listen to the DJ talk until he gets shot.
During your psychiatrist visit on the Prison stage, press the psychiatrist for answers, make sure to not accidentally demand your medication and then refuse your medication at the end.
During the Prison stage, hang up the phone and then kill everyone you encounter during the mission.
During the Psychiatrist session at the beginning of Chinatown, berate the Psychiatrist again, don't demand your medication, and say, "F@#$ You" at the end of the conversation.
During the Psychiatrist meeting at the end of Bunker Pt 2 and confront the Psychiatrist to start the secret boss battle!

When you visit the Psychiatrist during the last step, things will take a turn for the worse when Zero tries to hit him. Thus begins the battle.

The Psychiatrist

The Battle with the Psychiatrist will take place over several phases. Each is fairly different, but the first couple are simple (in concept, though not necessarily in action). It will take a little while to get this down, but once you're familiar and your reflexes are trained, you'll be ready to take him down.

Phase 1 - Graspy Hands

During the first phase, strange giant arms will come out of portals towards you. You'll need to hit the hand to make it recoil. Each operates separately so hitting one will NOT stop the other. I recommend only hitting one of the two each time because they tend to be fairly close and the second hand usually has a good chance of grabbing you in the air in between strikes.

If you get grabbed, you're not dead until the Psychiatrist comes down and zaps your head. If you struggle enough, you may be able to get yourself free, but the higher up you are, the less time you have to escape. For this reason, it's usually best to lure the hands down rather than rising up to fight them.

Phase 2 - Pew Pew

During this phase, the Psychiatrist will levitate upward and make copies of himself. He'll have several attacks during this phase, and the only way to damage him is to bounce his attacks back at one of his duplicates. You'll want to reach the last part of this phase (Scattershot) hopefully after just two Carousels and two Rain attacks.

Carousel - All of the duplicates will start circling around at the top middle of the screen. Try to stand right at the middle and wait for the first one to shoot. If you angle it up and slightly right, you'll hit the initial shooter. There will usually be three shots, but if you hit the second duplicate with the first shot, it will disappear and not attack, costing you a potential attack.

Rain - This usually follows a Carousel and leads back into Carousel until he's low on Duplicates. The Duplicates will form a line at the top of the screen and shoot straight down. Sometimes these shots are sequential, but sometimes they are semi-random. If you slow time, you can frequently figure out which pattern is being used and potentially reflect two shots.

Scatter Shot - This move happens once you get down to just a handful of duplicates, they'll teleport down to you in random patterns and then shoot at you. This is extremely difficult and you'll want to slow time to figure out how to dodge and reflect effectively. I recommend dodging all but the last shot and reflecting just that one unless you're feeling really confident as these shots travel very quickly, and your swing can easily put you in harm's way.

This last attack will repeat endlessly until either you're dead or the duplicates are.

Then things get weird.